The present invention relates to braking control apparatus for electric motor operated vehicles and more particularly to a braking control apparatus provided with a dynamic braking control circuit and well suited for use with battery operated fork lift trucks.
A type of braking control apparatus for electric motor operated vehicles in which regenerative braking is effected first and then plugging braking is effected in response to the movement of a forward/backward change-over device to the reverse position while an electric motor operated vehicle is in the forward running, is disclosed in U.S. Pat. No. 4,124,812 issued on Nov. 7, 1978 to Naito et al. In this apparatus, after the vehicle is stopped by the plugging braking in the above-mentioned manner, the vehicle is allowed to run backward by simply turning on the chopper again to start the control of duty cycle and thus the apparatus is well suited for use with the battery operated fork lift truck which is subject to frequent forward and backward movements.
To effect regenerative braking, it is necessary to change the connection of the main circuit from a motor circuit to a regenerative braking circuit by means of a regeneration braking contactor. This switching operation is carried out when the accelerator position is greater than a predetermined value and the duty cycle of the chopper circuit is approximately 100% and also the forward/backward change-over device is in operation.
As a result, in the above apparatus, when the forward/backward change-over device is first moved from the forward position to the neutral position and then to the forward position again or when the change-over device is first moved from the reverse position to the neutral position and then to the reverse position again, the main circuit connection is always changed to the regenerative braking circuit. Consequently, this apparatus has a drawback such that when the change-over device is moved from the forward (or reverse) position to the neutral position and then to the forward (or reverse) position, the motor drive (power running) circuit is not established immediately resulting in slow response in movement of the vehicle. Another drawback is that since the contactor must operate whenever the change-over device is changed from the forward to the neutral position and then from the neutral to the forward position or from the reverse to the neutral position and then from the neutral to the reverse position, the contactor is subjected to frequent operation so that its life is decreased.
Further, the transition from the regenerative braking to the plugging braking is carried out upon detecting that the duty cycle of the chopper in the regenerative braking mode has reached a predetermined value, such as, about 70 to 100%, and the regeneration contactor is closed after the chopper duty cycle has been reduced to zero % or after the operation of the chopper has been stopped. As a result, there is no braking force during the transition period (about 0.1 to 0.4 seconds) so that the coasting distance is increased and the driver may feel as if no deceleration were effected.